Make the Invisible Visible: Detecting IR Light with a Smart Phone

Felicia Cherry
Product Manager, Physical & Earth Sciences, Physics

The sun actually sends out more light than the part we see. Just past visible violet light is ultraviolet (UV) light, and just past visible red light is infrared (IR) light. Neither is visible to the human eye. Infrared light does not depend on visible light—IR light can be detected in dark or obscure conditions. Almost everything gives off infrared light as heat. If human beings had eyes adapted to see infrared, they might be overwhelmed by all the glowing matter around them. However, animals such as snakes, vampire bats, and beetles have developed organs and receptors to detect infrared heat signatures.

Though we cannot see infrared light, we can still use it. For example, a TV remote control uses infrared light to change the channels. You can use a digital camera to detect its presence. Aim a remote at a smartphone camera and press a button; you may see a light on your camera screen. The phone camera detects the light from the remote control and turns that signal into the visible light seen on the screen. When you press a button, do you see visible light at the end of the remote without using a camera? That does not indicate you have IR vision super power. That indicator light means the batteries in the remote are functioning properly.

Safety

Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules. Do not stare at an IR light source for prolonged periods of time.

Materials

Remote control with fresh batteries

Smart phone with both front and rear-facing cameras

Procedure

Part 1

Many of today’s smartphones have IR filters on the rear camera and little to no IR filter on the front-facing camera. It’s important to test this demonstration with both cameras before presenting it to the class. Only one camera should have the filter. This information is important for part 2. Should both cameras have an IR filter, you will need to use an older device.

Aim the remote at the device it operates. Use the remote with the device to show that the remote works.

Aim the remote at the class. Press the power button several times. Students should see no visible light.

Open the camera app on the smart phone. Switch the camera to front-facing.

Aim the remote at the front-facing camera. Press the button. A light should appear on the camera screen, proving that there is indeed a signal leaving the remote.

You may end the demonstration here or continue with the discrepant event in part 2.

Part 2

Again, aim the remote at the device it operates, and show that the remote works.

Using the rear-facing camera, repeat the steps in part 1.

No light will show up on the screen.

Switch back to the front-facing camera. Repeat the steps. The light will show up.

Challenge students to explain why. Answers will vary, depending on your students’ prior knowledge about properties of light waves, cameras, and smart devices.

Students may associate night vision equipment with infrared radiation. If so, be sure to point out that not all night vision devices rely on infrared radiation to “see in the dark.” Some devices amplify available ambient light, while others rely on thermal radiation.

Explanation

You established the fact that cameras can detect IR light. Why does one camera work but not the other one? No, the rear-facing camera is not broken. It’s designed to prevent infrared light from passing through the lens; therefore, no light is seen on the screen.

To improve photo quality and to make images appear more like what we actually see with our eyes, camera makers apply films and filters to block out infrared light. This process maximizes the amount of visible light that goes through the lens.

Extension activities

Research and report about the application of infrared light waves in modern times. Topics include but are not limited to: surveillance, prey detection by animals, heat-seeking devices, and architectural design.